The invention relates to a magnetic tape guiding device for use in a magnetic tape instrument having a helical guidance of the magnetic tape around a scanning device.
Magnetic tape instruments having a helical guidance device are used preferentially for the recording and playback of wideband signals, such as video signals. In such instruments, the magnetic tape is disposed around a transversally divided drum arrangement, in the division plane of which a magnetic head arrangement with one or more magnetic heads revolve inside to interact with the magnetic tape, so that the signals are recorded on the magnetic tape in a sequence of diagonal traces.
In order to be able to record a continuous sequence of signals without any gaps, it is necessary that at any given point in time at least one magnetic head of the magnetic head arrangement interacts with the magnetic tape.
In the case of a magnetic tape instrument with one recording, or playback head on the head-wheel, it is necessary that the magnetic tape be wound around the drum arrangement through an angle of about 360.degree.. If the head wheel includes two magnetic heads which are displaced by about 180.degree. with respect to one another, the gapless recording of the signals is obtained by having the tape disposed around the drum through an angle of 180.degree..
In order to utilize most of the recording surface of the magnetic tape, the tracks on the magnetic tape extend as much as possible from one edge of the magnetic tape to the opposite lying one, so that as much information as possible can be recorded per track segment. For certain case, auxiliary tracks and sound tracks are provided at one edge of the magnetic tape or at both edges. For maximum utilization of the recording surface on the magnetic tape in the case of a magnetic tape instrument with a single magnetic head, the magnetic tape is usually wound around the drum arrangement at a flat angle, whereas in the case of a magnetic tape instrument with two or more magnetic heads displaced at equal angles with respect to each other, the helical path of the magnetic tape around the drum arrangement becomes steeper. In each case, a vertical level displacement, approximately equal to the width of the magnetic tape, appears between the incoming and outgoing portions of the magnetic tape with respect to the drum arrangement.
This difference in vertical displacement between the incoming and outgoing portions of the magnetic tape makes the design of the instrument difficult as well as its construction and operation.
In general, it is preferable to have the drum arrangement and the tape guiding elements for the drawing and the transporting of the magnetic tape with their axes parallel to each other and perpendicular to a common base plate. For the base plate, a flat construction is preferred because it can be produced and machined relatively easily. As a result of the above mentioned vertical level displacement, e.g. the incoming magnetic tape end moves directly above the base plate towards the drum arrangement, whereas the outgoing magnetic tape end, after re-orientation is guided away in a plane tangent to the drum arrangement which is vertically higher from the base plate. Accordingly, the take-up reel must be arranged in a vertically displaced position with respect to the supply reel, the same applying to the guiding elements associated with the guidance of the in-going and out-going magnetic tape portions. Such an arrangement complicates the insertion process. In particular, it renders it difficult to use a fully automatic threading device.
There are known magnetic tape instruments designed to eliminate the aforementiond disadvantages by having the drum arrangement skewed with respect to the base plate so that the incoming and the outgoing magnetic tape portions exhibit the same vertical position with respect to the base plate. This arrangement has the serious disadvantage that the driving device for the magnetic head arrangement must likewise be inclined with respect to the base plate, thereby greatly impeding the transmission of the required angular forces, from the head arrangement to the reels or the reverse by means of simple belt drives. In the case of a multi-motor arrangement, that is, separate drive motors for the magnetic head arrangement and the reels, these difficulties do not exist, but such embodiments are relatively expensive.
Generally, one drawback in such magnetic tape instruments is that the magnetic tape must run through the guiding device and the drum arrangement with a high degree of accuracy in order to guarantee the reproducibility of the recorded signals and that simultaneously any kind of localized or one-sided stretching must be avoided in order to eliminate permanent deformations which could adversely affect the playback capability of the magnetic tape.
From the DT-PS No. 1,175,726 (corresponding to the U.S. Pat. No. 3,159,501) a device is known for the guidance of a magnetic tape in a magnetic tape instrument having a rotary head arrangement with two magnetic heads displaced by 180.degree. with respect to one another. In this instrument, the portion of the magnetic tape which is in operating in connection with the rotary head arrangement encircles approximately half the circumference of the drum arrangement. This device for the guidance of the magnetic tape includes an external tape guiding system with two rounded guide members which are axially displaced with respect to each other and each is disposed tangent to a respective plane tangent to the drum arrangement. In this manner, the magnetic tape runs outside of the guide members in planes which are perpendicular with respect to the axis of the drum arrangement, so that the axes of the supply and take-up reels can be arranged parallel to the axis of the rotary head arrangement. This arrangement permits the axis-parallel arrangement of the take-up, and supply reels, as well as of drum arrangement and rotary head, yet it does not eliminate the disadvantages of the vertical position displacement of the incoming and outgoing magnetic tape as they relate to the threading-in process such as one with automatic threading.